The invention relates to a vibration isolation device and more specifically the invention relates to a vibration isolation device for an aircraft engine oil tank wherein the isolation device includes means for decreasing the mount stiffness when loads above a threshold load are experienced.
Aircraft jet engines include a turbine fan or blower that draws ambient air into the engine for compression and combustion by the engine. The turbine fan or blower is shrouded by a casing. An engine oil tank is attached to the engine fan casing by a rigid bracket and pin connection that is fixed to the casing and the bracket is in turn made integral with a vibration isolator that is fixed to the exterior of the oil tank housing. The conventional oil tank vibration isolator is relatively rigid with substantially constant stiffness. As a result, the isolator operates in a manner similar to a hard mount. In such conventional aircraft engine isolation systems, during normal loading and operating conditions the conventional isolator limits the transmission of engine vibratory loads to the oil tank. Typically, such loads are high frequency loads, having relatively low amplitude or magnitude. During an engine blade out event or during any period when the load applied to the isolator is applied at a frequency lower than normal operating speeds and is of an amplitude or magnitude above the normal operating load condition, the rigid mount may be unable to limit transmission of such large applied loads to the oil tank and as a result, the larger than typical loads may damage the oil tank.
The foregoing illustrates limitations known to exist in present isolators. Thus, it is apparent that it would be advantageous to provide an alternative oil tank isolator that is designed to absorb engine loads under typical engine operating conditions and also larger vibratory loads that are experienced during an engine fan blade out event for example. Accordingly, a suitable alternative isolator is provided including features more fully disclosed hereinafter.
In one aspect of the present invention this is accomplished by providing a vibration isolation device located between a first member and a second member, the isolation device having a first isolation device stiffness supplied when loads applied to the isolation device are below a predetermined threshold load, the isolation device further comprising a second isolation device stiffness supplied when the load applied to the isolation device is at least equal to the threshold load, the vibration isolation device further comprising a fuse means for altering the mount stiffness to the second isolation device stiffness when the threshold load is applied to the device, the isolation device being movable relative to the first member in a plurality of directions after the fuse is actuated.
In the vibration isolation device of the present invention the means for decreasing the device stiffness is a shear pin that includes at least one shear location along the length of the pin. The present invention comprises two shear locations and both of the shear locations comprise an annular groove with a semielliptical cross section.
The vibration isolation device of the present invention includes a housing defining a chamber with a spring member located in the housing. The spring member comprises a first plate member and a second plate member that define an inner chamber, a first shim member and a second shim member and a first resilient member joining the first shim and first plate and a second resilient member joining the second shim and second plate.
The foregoing and other aspects will become apparent from the following detailed description of the isolation device of the present invention when considered in conjunction with the accompanying drawing figures.